# The Expansion of the Universe ## expand\_more ## From the Hubble Law to the Hubble Space Telescope, and Beyond! ### **About Hubble’s Law** Hubble’s Law shows our Universe to be ever-changing, expanding its limits in space and time simultaneously. In 1929, the American Astronomer Edwin Hubble made a simple graph showing that galaxies farther from each other move away from each other systematically faster. He drew a simple straight line on the velocity vs. distance plot shown below, summarizing the trend that now bears his name as Hubble’s Law, represented today as: ***Velocity = (Hubble’s Constant) x Distance*** This law implies that the Universe is expanding, and Hubble’s Constant measures how fast. These findings were revolutionary in the 1920s. ### **How do Hubble’s galaxies look through the Hubble Space Telescope?** In the interactive version of Hubble’s graph below\*, yellow circles mark individual galaxies, and green labels show identifiying [NGC numbers](https://en.wikipedia.org/wiki/New_General_Catalogue) (or names) for each galaxy. Click on a yellow circle below to learn more about each galaxy and see a Hubble Space Telescope image of it superimposed on all-sky images. Online graph URL goes to: Embed \*This graph is based on his original [published](https://ui.adsabs.harvard.edu/abs/1929pnas...15..168h/abstract) figure. ### **How correct is this graph?** A1. Hubble’s mis-labeling of velocity units as “KM” instead of “KM/S” has not been corrected here. A2. The actual “Hubble constant” (slope) one would derive from this plot is about 500 km/s per million pc (Mpc), but the [modern value is](https://en.wikipedia.org/wiki/Hubble's_law) about 70 km/s/Mpc. 3.) Hubble’s measurements of “velocity” did not use the same system as in modern times, so a couple of the galaxies may be mis-labeled here. Please let us [know](mailto:wwtambassador@gmail.com) if you figure out which they are! ### **Where can I find more information about Hubble’s graph and Law?** The original version of Hubble’s plot appeared in his 1929 paper, [A Relation between Distance and Radial Velocity among Extra-Galactic Nebulae](https://ui.adsabs.harvard.edu/abs/1929pnas...15..168h/abstract). An explanatory article about [Hubble’s Diagram and Cosmic Expansion](https://www.pnas.org/content/101/1/8) was published by Robert Kirshner in 2004 in PNAS, and its a great resource for learning more about Hubble’s diagram and its significance. For reference, the expert-level caption for Hubble’s original graph given in Kirshner’s PNAS article reads: *“Radial velocities, corrected for solar motion (but labeled in the wrong units), are plotted against distances estimated from involved stars and mean luminosities of nebulae in a cluster. The black discs \[encircled in yellow in this site’s interactive plot\] and full line represent the solution for solar motion by using the nebulae individually; the circles and broken line represent the solution combining the nebulae into groups; the cross represents the mean velocity corresponding to the mean distance of 22 nebulae whose distances could not be estimated individually.”* ### **Caught your Interest?** [**KEEP READING**](/more-about-hubbles-law) **about Hubble's Law!** Sort## **What's next for this project?** **We are planning to showcase Hubble’s story in an introductory data-science curriculum, using a simplified interface of NSF & NASA-supported glue and WWT software. Educators with an interest in data science who would like to know more, please contact .**